Thirty years ago two cosmonauts commuted from one space station to another, the only time that’s ever been done.

Thirty years ago today, a pair of Soviet cosmonauts made an unprecedented journey between two space stations, under circumstances befitting a science fiction novel. Surprisingly, a number of key details of the dramatic saga only recently became known.

During their 125-day mission in 1986, Leonid Kizim and Vladimir Soloviev were the first people to enter the new Mir space station. They then transferred for several weeks to the aging Salyut-7 orbital lab, which had been hastily evacuated several months earlier after its commander Vladimir Vasyutin fell seriously ill with a prostate infection. As a result, a number of major experiments onboard the station had been left unfinished.

Salyut-7 had been in orbit since April 1982, and was due to be succeeded by the much larger, multi-module Mir station. But after Vasyutin’s crew left in a hurry on November 21, 1985, engineers pondered whether to postpone the launch of Mir and send a fresh crew to the old station to finish the work there. That would have required using up the only remaining Soyuz T transport spacecraft, though. At the time, the new-generation Soyuz TM was not expected to be certified to carry cosmonauts until the end of 1986.

Instead, engineers came up with the idea to use the same Soyuz T-15 spacecraft for both missions, by shuttling the vehicle between the two stations. Kizim and Soloviev, who already had spent 237 days on Salyut-7 in 1984 and knew every corner of the old lab, were chosen for the difficult mission.

To make it possible for the cosmonauts to taxi between the two stations, the first module of Mir was inserted into the same orbital plane with Salyut-7. The so-called core module of Mir blasted off from Baikonur on a Proton rocket on February 20, 1986.

The stage was now set for the historic transfer, but on March 8, during a routine testing of the rendezvous system on Salyut-7, one of the countless switches in the station’s electronic brain stuck in the “off” position. This seemingly minor glitch sent a strong electric current to critical propulsion valves in the lab and thwarted all attempts to activate its engines, leaving Salyut-7 in a state of virtual paralysis.

In the last few days before their launch, Kizim and Soloviev had to cram in extra training so they could troubleshoot the problem on Salyut-7. Finally, on March 13, Soyuz T-15 blasted off into orbit. After months of training and an extended 50-hour chase of the Mir station to conserve fuel for future maneuvers, the crew was ecstatic to finally see their first destination.

“When we approached, [Mir] was like a giant white-wing seagull soaring above the Earth,” Kizim radioed to the ground, as Soyuz T-15 intercepted the new station at an altitude of around 350 kilometers on March 15. For the next month and a half, the duo worked on activating Mir.

Then, on May 5, they climbed back inside their Soyuz T-15 to begin chasing down Salyut-7, which was circling the Earth in a higher orbit more than 3,000 kilometers ahead. A day later, they successfully docked at the tail end of the paralyzed station. “It is like I never left,” Kizim joked over the radio after re-entering his home of almost two years earlier.

After carefully testing Salyut-7’s atmosphere with a gas analyzer, the first order of business (although it wasn’t disclosed at the time) was to try to find the problematic switch in the electronic jungle of Salyut-7. But despite the cosmonauts’ best efforts, they couldn’t pinpoint the offending circuit. Plan B was to insert a cable bypass that would get around the stubborn “off” command, and that technique worked. Immediately, the power consumption onboard the Salyut went down. The station still couldn’t maneuver on its own, but a large space tug named Kosmos-1886, which had been docked there since October 1985, could take over most of the propulsion responsibilities.

Still, engineers were concerned about possible new short-circuits and faulty commands, especially since Kizim and Soloviev were scheduled to do spacewalks. What if some errant command locked the cosmonauts outside the station? After some debate and testing, flight controllers allowed the crew to proceed with two spacewalks, which featured laborious deployment of a 12-meter experimental truss. By the time they finished, Kizim and Soloviev had logged more than 30 hours outside, without any serious problems.

After completing most of the work onboard Salyut-7, the duo shuttled back to Mir at the end of June. There were no plans to occupy the older station again, so before leaving the cosmonauts packed up all the useful hardware they could carry, completely stuffing the Soyuz habitation module with 400 kilograms of cargo, including a guitar. There had once been proposals to send the Soviet Buran shuttle to keep Salyut-7 going, but that never materialized. “So, are we the last [to leave]?” one of the cosmonauts radioed to mission control before closing the station’s hatch for the final time.

Abandoning Salyut-7 had its own risks, because nobody could predict what the capricious computers might do. Instead of deorbiting the lab, a decision was made to boost Salyut-7 to a higher orbit, where it could stay for around a decade in an improvised long-duration test.

Kizim and Soloviev arrived back at Mir on June 26. “Everything seems in order,” Kizim radioed to the ground after re-entering the core module, “I guess nobody dropped by while we were away.”

The crew’s return to Earth was originally scheduled for August 20, based on the expiration date of their Soyuz T-15. But shortly after the cosmonauts re-entered Mir, mission managers began questioning the wisdom of keeping the overworked crew in orbit. All key tasks of the mission had been completed, and the launch of the Kvant module, which would deliver the first major scientific payload to Mir, had already slipped to the next year.

In the end, it was decided that the crew would return on July 16, 1986, after four months in space. The Soyuz descent module successfully touched down 55 kilometers northeast of the Kazakh town of Arkalyk at 16:34 Moscow time, ending the unique mission of Soyuz T-15—the only time in history that a space crew has shuttled between two “homes” in orbit.

This documentary (in Russian) includes lots of video from the mission: